Future generations of high-capacity packet switches, cross-connects and other telecommunication apparatus will likely rely on internal optical switching technologies to overcome current limitations facing high-speed and high-capacity electronic switching, routing and transport. Additionally, there are numerous other applications for optical wavelength switching technologies that have been discussed recently, including, for example, future optical metro or access networks, optical memory access, optical super-computing, and optical processing in general. A common factor among these applications is that a particular function (e.g. port switching, packet routing, memory access, etc.) depends on the wavelength of an optical input signal. For example, in a router, the function of directing a signal to a particular output port depends upon the wavelength of the signal. These functions are referred to herein as “wavelength-dependent functions.”
Of particular interest herein are wavelength-dependent functions for signals which are “time division multiplexed” (TDM) on a single input line, meaning that different wavelength signals are interposed or “slotted” as a function of time on a single transmission line. This is a well known technique. For port routing or addressing based on a TDM signal, two main concepts should be distinguished. The first is a point-to-point transmission, in which the TDM signal is generated at the transmitter side by a time division multiplexer (TDM) transmitter. The second is a broadcast-and-select transmission, in which optical signals of varying wavelengths are combined from a number of fixed-wavelength transmitters. The broadcast-and-select transmission is sent to all ports, at which a bandpass receiver selects certain wavelengths to generate a TDM signal.
In a point-to-point transmission, the TDM transmitter generates a TDM signal. A typical TDM transmitter comprises a bank of lasers in which each laser generates continuously a particular wavelength output. Such lasers are referred to herein generally as “wavelength-dedicated devices.” These wavelength-dedicated devices are coupled through gates to a common output line. If the gate is opened, the output of that particular laser is allowed to reach the common line, otherwise the output is block. In this way, the gates are turned on and off selectivity in response to electrical input signals to output a desired TDM optical signal on the output line. Since gating can be performed at very high rates and since dedicated wavelength lasers have extremely stable and reliable output, this TDM transmitter configuration has become widely used.
In a broadcast-and-select transmission, a bandpass receiver receives a wavelength division multiplexed (WDM) signal and outputs a TDM signal. A traditional bandpass receiver is similar in many resects to the TDM transceiver. A receiver comprises a passive optical demultiplexer to split the multi-wavelength signal into signals having discrete wavelengths. This demultiplexer comprises a dedicated filter for each signal wavelength. Like the TDM transmitter, the output of each filter is coupled to a common line. The TDM signal on the common line is controlled by switching on and off gates coupled to each filter. If the gate is opened, the output of that particular filter is allowed to reach the common line, otherwise the output is block. As with TDM transmitters, since gating can be performed at very high rates and since dedicated wavelength filters have reliable output, this TDM receiver configuration has become widely used.
Despite the popularity and reliability of the TDM transmitters and receivers described above, the fact that dedicated-wavelength devices are used makes these systems expensive and bulky. Therefore, there is a desire to replace these wavelength-dedicated devices with wavelength-tunable devices, such as tunable lasers and filters. It is anticipated that, by replacing these dedicated components in favor of more-versatile, wavelength-tunable devices, the transmitters and receivers can be simplified and miniaturized.
Unfortunately, tunable lasers and filters do not change their output fast enough to meet the requirements of many of today's routing and switching networks, much less future needs, particular in the computing and data access applications. Specifically, the wavelength-tunable devices require time to stabilize the frequency of their output once they initiate a change in wavelength output. This time is referred to herein as “settle time.” For example, today's fastest tunable laser requires about 50 ns to achieve a stable signal of ±3 GHz. Such a delay may be insignificant for many applications, for example, a network based on the SONET (synchronized optical network) standard has a tolerance of about 50 ms, which is a great deal higher than this settle time, so the wavelength-tunable device can be essentially taken off-line while it is stabilizing without negatively affecting the network. However, other optical switching applications, such as high-capacity packet switching, are not as tolerant.
Thus, there is a need for TDM transmitters and receivers which use wavelength-tunable devices, but which are quick enough to meet existing and future optical switching needs. The present invention fulfills this need among others.